Laser diodes, in particular made of GaAs/AlGaAs, InGaAsP/InP and other ternary or quaternary alloys, have been intensely investigated in the last few years. The different designs of these lasers result in devices with highly efficient, extremely low thresholds and being capable of high power operation.
Especially for optical fiber communication, but also for other applications, the InGaAsP/InP material system received the most attention. This change of direction, from the well established AlGaAs/GaAs lasers to the InP based long wavelength lasers, emitting in the range of 1.3 .mu.m-1.5 .mu.m, was initiated by the great demand for lasers being suited for optical communication via silica fibers. The refractive index of this fiber material is wavelength dependant, giving the rise to a material dispersion which passes through zero at approximately 1.3 .mu.m. This wavelength can be achieved by InGaAsP/InP laser diodes, as for example reported on in "Semiconductor Lasers for Long-Wavelength Optical-Fiber Communications Systems", M. J. Adams et al., published by P. Peregrinus Ltd., London, UK., 1987.
A typical InGaAsP laser diode with ridge waveguide structure is disclosed in "Performance of an Improved InGaAsP Ridge Waveguide Laser at 1.3 .mu.m", I. P. Kaminow et al., Electronics Letters, Vol. 17, No. 9, April 1981, pp. 318-320. The structure of this separate confinement double heterostructure (SCH) ridge waveguide laser is illustrated in FIG. 1A, the thickness of the layers being exaggerated for the sake of clarity. The laser 10 consists of a separate confinement heterostructure (SCH) 12-14 which is grown on top of a n-doped InP substrate 11. The heterostructure comprises a lower n-doped InGaAsP cladding layer 12 (E.sub.g =1.13 eV), an undoped InGaAsP active layer 13 (E.sub.g =0.95 eV), and an upper p-doped InGaAsP cladding layer 14 (E.sub.g =1.13 eV). The ridge itself consists of an InP layer 15 (E.sub.g =1.35 eV) being p-doped and which is covered by a p-doped InGaAsP cap layer 16 and an Au/Zn alloyed contact layer 17. The whole structure, with the exception of the ridge, is covered by a Si.sub.3 N.sub.4 insulation layer 18. A Ti/Ag, Au metallization 19 provides for an electric contact to the alloyed contact layer 17 on top of the ridge.
As can be seen from FIG. 1B, which shows the bandgap (E.sub.g) vs. position (x) diagram of the respective structure 10, the active layer 13 with a bandgap of 0.95 eV is surrounded by higher bandgap cladding layers 12 and 14 with E.sub.g =1.13 eV. The energy barrier of 0.18 eV (30% of the total bandgap step between InP, 1.35 eV, and InGaAs, 0.75 eV) is not very high and does not provide for efficient electric carrier confinement. The real energy barrier in the valance band is about 60% of said 0.18 eV step and the barrier in the carrier band about 40% of 0.18 eV. The higher these energy barriers are, the better are the carriers confined inside the active layer 13. Optical confinement is provided by the refractive index step between the cladding layers 12 and 14 and the surrounding InP layers 11 and 15, as shown in the refractive index (n) vs. position (x) plot of FIG. 1C. The higher tile energy barriers are, i.e. a structure providing for efficient carrier confinement in the active layer, the worst is the optical confinement because said refractive index step decreases with increasing bandgap step and the maximum bandgap discontinuity remains constant only depending on the materials used.
Typical for this quaternary InGaAsP alloy is that the bandgap (E.sub.g) can only be varied between 1.35 eV (InP) and 0.75 eV (InGaAs). Any improvement of the carrier confinement by providing for higher energy barriers has a detrimental effect on the optical confinement. None of the structures known in the art, some of them being listed below, provide for a solution of this conflict:
"Low Threshold InGaAsP Ridge Waveguide Lasers at 1.3 .mu.m", I. P. Kaminow et al., IEEE Journal of Quantum Electronics, Vol. QE-19, No. 8, August 1983, pp. 1312-1318; PA1 "Continuously Graded-Index Separate Confinement Heterostructure Multiquantum Well Ga.sub.1-x In.sub.x As.sub.1-y P.sub.y /InP Ridge Waveguide Lasers Grown by Low-Pressure Metalorganic Chemical Vapor Deposition with Lattice-Matched Quaternary Wells and Barriers", M. J. Ludowise et al., Applied Physics Letters, Vol. 57, No. 15, October 1990, pp. 1493-1495; PA1 "Strained Multiple Quantum Well Lasers Emitting at 1.3 .mu.m Grown by Low-Pressure Metalorganic Vapor Phase Epitaxy", D. Coblentz et al., Applied Physics Letters, Vol. 59, No. 4, July 1991, pp. 405-407.